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DESCRIPTION

G.729 is an umbrella
of vocoder standards. The G.729 vocoders perform voice compression
at bit rates that vary between 6.4 and 12.4 kbps. The figure below shows
an example of the G.729 vocoder connected to a digital communication channel. The input speech is fed into the G.729 encoder as a stream of16-bit PCM
samples, sampled at a rate of 8000 samples/second. The G.729 encoder compresses
the data into the Encode Stream. The encoder also outputs the DTX status,
which is discussed later in this data sheet. The digital channel carries
the data stream and DTX status to the decoder, which regenerates a representation
of the original speech, and outputs it as the output speech – again
as 16-bit PCM at a sampling rate of 8000 samples/second. Since G.729 is
a uses lossy compression, the output speech is not identical to the input
speech.

The decoder is
also fed a frame erase flag, which is an indication that the decode stream
has temporarily been corrupted. The decoder is able to “smooth over” the
output , doing its best to conceal the loss of data and minimize the loss
in voice quality. This process is known as packet loss concealment (PLC).
It works surprisingly well even under high packet loss rates.